This invention relates to field emission guns employing an emission source, an apertured beam accelerating electrode and a Wehnelt or control electrode in the vicinity of the source and responsive to regulation signals for modulating the field emission from the source. The invention relates specifically to the geometry of the field emission triode comprising the emission source, Wehnelt electrode, and accelerating electrode and to the precise dimensions required to produce a stabilized focus as the regulation signal is continuously being varied in polarity and amplitude to maintain a constant beam current.
In field emission beam current compensation systems employing the above mentioned triode elements, the continuously varying field strength in the vicinity of the emission source affects the emitted beam to produce an undesirable shifting in the axial position of the virtual emission source as the regulation signal to the Wehnelt is varied. This shifting effect results in an instability of the focus of the beam and tends to diminish in value the advantages obtained by the beam noise compensation system.
Focus instability in the triode regions of field emission gun system are well recognized and treatises on the problem have been published by J. R. A. Cleaver in the "International Journal of Electronics", Volume 38, No. 4, at Pages 531-540, and also by S. Nomura et al in the "IRTRI Proceedings" (1973) at Page 35. S. Nomura et al was also granted a U.S. Pat. No. 3,925,706, which describes a beam compensation system employing triode elements in which the accelerating electrode is provided with a novel semispherical surface for improving the electron optical characteristics of the system. This approach, as well as other known prior art devices developed for improving the electron optical characteristics in field emission systems, having relied on the theory that there were three variables that must be properly coordinated to obtain the desired improvement. These variables are (1) the spacing between the accelerating electrode and the Wehnelt or control electrode, (2) the spacing between the emission source and accelerating electrode, and (3) the aperture diameter in the Wehnelt electrode. While proper selection of these three dimensions will greatly improve beam focus stability in the field of emission guns, there remains a considerable amount of focus instability so that the virtual source of the beam, as viewed from outside of the emitting region, appears to shift along the beam axis as the continuously varying regulation signal is applied to the Wehnelt electrode.
We had discovered that for precise beam focus stability, an important fourth variable, the diameter of the aperture in the accelerating electrode, must be accurately selected. Thus the invention is for a field emission triode structure which, when viewed from outside of the emitting region, appears to have a non-shifting virtual emission source irrespective of continuously varying regulation signals applied to the Wehnelt electrode. This non-shifting virtual source is achieved by the proper selection of the dimensions for (1) the spacing between the accelerating electrode and the Wehnelt electrode, (2) the spacing between the emission source and the accelerating electrode, (3) the aperture diameter of the Wehnelt electrode, and (4) the diameter of the aperture in the accelerating electrode.